a. Field of the Invention
The invention concerns a method for the control of the focusing of ultrasonic field and apparatus for performing the method.
b. Description of the Prior Art
Focussed ultrasonic fields are used, for example, in medicine to break up concretions present in the body, e.g., kidney stones. Such a procedure is advantageous in the avoidance of surgical interventions, the introduction of probes into the patient's body and the dangerous infections connected therewith.
Equipment for the shattering of a concretion without direct contact is known from EP-A2 0 133 665, where the approximately level ultrasonic wave produced in a shock tube is focussed on the target area with an acoustic lens or a system of acoustic lenses. A coupling medium is located between the lens or the system of lenses and a copper membrane serving as ultrasonic radiator. The shock tube can be aimed at the concretion with the aid of a holder so that the focal point of the lens on the side of the image lies in the concretion. The lens can also be shifted along the axis of the shock tube with the aid of a fine adjustment means for the exact positioning of the focal point.
A disadvantage of the mentioned equipment consists in its inability to detect changes in the focusing of the field. For example, a change in the position of the focal point or a change in its lateral and axial position, during the use of the equipment. For the determination of the focusing of the field, the instrument must be removed from the patient and checked in a suitable device. Such changes in the focal properties may occur, for example, through a change in the wave field emitted by the ultrasonic transmitter or through a mechanical change in the adjustment of the imaging system.